In some wireless communication systems, a user equipment (UE) wirelessly communicates with a base station. A wireless communication from the UE to the base station is referred to as an uplink communication. A wireless communication from the base station to the UE is referred to as a downlink communication. Resources are required to perform uplink and downlink communications. For example, a UE may wirelessly transmit data to a base station in an uplink communication at a particular frequency during a particular time slot. The frequency and time slot used are examples of resources.
An uplink frame is a set of time/frequency resources allocated for uplink communications from UEs to a base station. One example of an uplink frame is an uplink orthogonal frequency-division multiple access (OFDMA) frame.
In some wireless communication systems, if a UE wants to transmit data to the base station, a demand assignment is performed, as follows. The UE first sends a message, to the base station, requesting uplink resources. The base station responds to the request by scheduling the UE on some time/frequency resources of an uplink frame. The base station sends a message to the UE indicating the time/frequency resources on which the UE has been scheduled. The UE then sends the uplink transmission using the time/frequency resources. The time/frequency resources may be referred to as a resource block.
Multiple access occurs when more than one UE is scheduled on the same uplink frame. Each UE uses a different dedicated group of time/frequency resources of the uplink frame to transmit data to the base station. The base station decides, on a frame-by-frame basis, which UEs will be scheduled on which resources of the uplink frame. A UE may be granted resources in one uplink frame, but not in another uplink frame.
Multiple access using demand assignment is referred to as demand assignment multiple access. Demand assignment multiple access may not be desirable for all UEs being served by a base station. For example, a particular UE may have bursty traffic that needs to be transmitted to the base station with low latency. The delay incurred by a demand assignment procedure may introduce an unacceptable amount of latency for some traffic types or applications.